In 1854, a young British naturalist named Alfred Russell Wallace was traveling through the East Indies observing and collecting specimens to study natural history. Wallace collected nearly 126,000 different specimens (with over half being beetles). Many of the specimens were completely new to science, and the exciting discoveries offered a view into the incredible biodiversity housed in the Malay Archipelago.
Throughout his observations and collection of specimens, Wallace observed that animals living on western islands shared characteristics with many species in Asia, indicating a shared origin. In contrast, he noted that islands to the east of the line housed fauna similar with Asian and Australian animals. The pattern was consistent and striking. Wallace also noted that the flora on the island did not follow this same pattern- basically, the plant life on the islands was similar but the animals were quite different. What was going on?
Wallace had discovered bio diverse hotspot indicated a connection between the Australian and Asian continents, further explained by ancient sea levels and the location of continental shelves. During the ice ages, sea levels were lower, which allowed continental shelve to be exposed and simultaneously provide a land bridge for animal migration. The majority of birds, mammals and reptiles found on the Indonesian islands that Wallace explored also had the ability to cross the straits but would not cross a large, open ocean. In this case, animals had migrated by crossing the straits but were not able to migrate over the vastness of an open ocean. The results were islands with distinct animal populations originating from, at one point, a larger connected continent.
In 1859, Wallace’s observations led him draw a line through the Indonesian islands separated by the unusual fauna diversity. This line became known as the Wallace line, and runs through the Lombok strait between Lombok and Bali, as well as between Borneo and Sulawesi. It continues to be recognized and used by scientists to this day.